JPH0344275B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a novel method for manufacturing a nuclear reactor fuel cladding tube, and particularly to a method for manufacturing a zirconium alloy nuclear reactor fuel cladding tube. [Background of the Invention] Zirconium-based alloys are used for fuel cladding tubes, fuel channel boxes, etc. of nuclear power plants because of their excellent corrosion resistance and extremely small neutron absorption cross section. These structures are exposed to neutron irradiation for a long period of time in a nuclear reactor and are also exposed to high-temperature, high-pressure water or steam, so as corrosion progresses, a zirconium oxide film is formed on the surface. Furthermore, spotty white oxides called nodular corrosion may be formed on the surface. These speckled white oxides become coarser as the corrosion reaction progresses, and sometimes peel off. When thinning of a member occurs due to such abnormal corrosion, the strength of the member decreases, raising concerns about the safety and reliability of the reactor internal structural member. Methods to prevent abnormal corrosion caused by this nodular corrosion are being studied. Among zirconium-based alloys, Zircaloy-2
(About 1.5% Sn, 0.1% Fe, 0.1% Cr and 0.05% Ni in Zr)
Zircaloy-4 (approximately 1.5
%Sn, 0.2%Fe, 0.1%Cr) is α+
It is known that corrosion resistance is significantly improved by rapid heating to the β phase or β phase temperature range, followed by rapid cooling (hereinafter referred to as β quenching) (Japanese Patent Laid-Open No. 58-22364, 25466, Publication No. 25467). Fuel cladding for nuclear reactors has two main roles. The first is to prevent chemical reactions due to direct contact between nuclear fuel and coolant, or between nuclear fuel and moderator. The second is to prevent radioactive fission products generated from the nuclear fuel from escaping into the coolant or moderator. However, the cladding becomes more brittle due to interaction with nuclear fuel and fission products, and further due to neutron irradiation, making it more susceptible to cracking. This tendency is exacerbated by local mechanical stresses due to differential thermal expansion between the fuel and the cladding. When nuclear fission products, especially iodine and cadmium, are generated during the operation of a nuclear reactor, and at the same time local stress as described above is applied, stress corrosion cracking may occur in the cladding tube. As a measure to prevent such stress corrosion cracking, it is known to provide a pure metal layer between the fuel and the cladding tube. In particular, as a composite type cladding tube in which pure zirconium is lined inside the cladding tube,
51-69795 and 54-59600 are known. The thickness of the pure zirconium layer is approximately 5 to 30% of the cladding wall thickness.
It is. Pure zirconium maintains its softness during use compared to zirconium alloys, so it reduces local stress acting on the cladding and prevents the stress corrosion cracking described above. As mentioned above, the outside of the fuel cladding tube has corrosion problems due to high temperature water and water vapor, that is, the tube thickness is reduced due to nodular corrosion, and the inside is affected by the gases released from the combustion of fuel pellets (for example, iodine) and the burning of the fuel pellets. The problem is thought to be stress corrosion cracking due to the bulging load associated with this process. Regarding this problem, as mentioned above, countermeasures have been considered for the former, such as heat treatment, and for the latter, a composite structure. However, when the conventional β-quenching heat treatment method is applied, the nodular corrosion resistance of the outer surface of the tube in contact with reactor water is improved, but the inner surface of the tube tends to be more susceptible to stress corrosion cracking. The reason for this is β
This is thought to be because the acicular structure formed by quenching is hard and has low ductility. Furthermore, even after cold working and annealing, the quenched material showed more susceptibility to stress corrosion cracking than the normally annealed material. In addition, when beta-quenching is applied to a composite cladding tube whose inner surface is lined with pure zirconium for the purpose of improving stress corrosion cracking resistance, the solute elements of the zircaloy, such as Sn, Fe, Cr, and O, become pure during high-temperature heating. It is thought that it diffuses into zirconium and reduces SCC resistance. [Object of the Invention] An object of the present invention is to provide a method for producing a fuel cladding made of a zirconium-based alloy that has excellent nodule corrosion resistance in high-temperature water or steam, and at the same time has low susceptibility to stress corrosion cracking due to iodine, etc. be. [Summary of the Invention] The present invention provides a method for forming a β-phase or α+ phase of a zirconium-based alloy after final hot plastic working.
A quenching process is carried out by heating to the β-phase temperature region and then rapidly cooling, followed by cold plastic working and annealing for at least one time.
In the method for manufacturing a cladding tube, at least the annealing after the cold plastic working after the quenching treatment is performed while the outer surface of the cladding tube is cooled and the inner surface of the cladding tube is heated to a temperature higher than the recrystallization temperature of the zirconium-based alloy. It is characterized in that it is carried out by heating to. Further, in the present invention, after the final hot plastic working of the zirconium-based alloy, a quenching treatment is performed to heat the zirconium-based alloy to the β phase or α+β phase temperature range and rapidly cool it, and then cold plastic working and annealing are performed at least once. In the method for manufacturing a cladding tube, the quenching treatment involves heating the entire cladding tube to the β phase or α+β phase temperature range and rapidly cooling it, or heating the outer surface of the cladding tube to the temperature range while cooling the inner surface of the cladding tube. The inner surface of the cladding tube is annealed by heating to a β phase or α+β phase temperature region and then rapidly cooled, and at least the annealing after the cold plastic working after the quenching treatment is performed on the inner surface of the cladding tube while cooling the outer surface of the cladding tube. A method for producing a cladding tube for a nuclear reactor fuel, characterized in that heating the above-mentioned zirconium-based alloy to a temperature higher than the recrystallization temperature. Zirconium-based alloy contains 1 to 2% Sn by weight,
Fe0.05~0.2%, Cr0.05~0.2%, Mi0 or 0.03~
0.1%, with the remainder preferably consisting essentially of zirconium. The reactor fuel cladding tube obtained by the manufacturing method of the present invention has almost no precipitates on the outer surface, so it has excellent corrosion resistance against high-temperature, high-pressure water, and the inner surface is soft, so it is resistant to stress corrosion cracking. Excellent quality. Among the alloying elements mentioned above, Fe, Ni and
It is preferable to control the amount of precipitates so that the total solid solution amount of Cr is 0.28% or more. Precipitates that form in zirconium-based alloys are
ZrC, ZrCr ₂ , Zr (Fe, Cr) ₂ , Zr (Fe, Ni) ₂ , Zr ₂
(Fe, Ni), etc. Furthermore, an alloy containing Nb is applied as the zirconium-based alloy. Once quenched, the stress corrosion cracking resistance of zirconium-based alloys tends to be higher than that of unquenched parts, even after cold plastic working and annealing. It is important not to take history. Further, if the inner side has a hardened structure, it is preferable to perform sufficient annealing to restore the recrystallized structure. When quenching is performed from a temperature range that includes the β phase after hot extrusion, the inside of the raw tube that is being manufactured is cooled using water, hot water, steam, gas, a salt bath, or a cooling mold, and the inside of the tube is heated. remains at a low temperature in the alpha phase region of the alloy. Preferably, the inside temperature does not exceed 600°C. In other words, the outside of the raw pipe,
Hardening is performed while creating a temperature gradient between the two to prevent the inside temperature from rising as much as possible. At this time, the outside temperature is heated to a temperature that includes the β phase.
In Zircaloy-2 or Zircaloy-4, the heating temperature at which the β phase appears is about 900°C. When heating to the (α+β) two-phase region, heat to 900 to 1000°C, and if heating to the β-phase region, heat to 1000°C or more. Heating can be achieved by high frequency, electrical heating, electron beam, and laser beam heating methods, but the high frequency heating method provides a more stable hardened structure. During quenching, it is preferable that the temperature in the inner 1/3 region is 600°C or lower. This is in consideration of the reduction in wall thickness due to subsequent cold plastic working, machining, etc. By this treatment, a β-quenched tube is obtained in which the outer layer of the tube has a quenched structure and the inner layer has a structure as hot extruded or an annealed structure. Hardening treatment with a temperature gradient is also effective for billets with a metal barrier such as pure zirconium on the inside of the tube. After quenching, the raw tube is subjected to cold plastic working and annealing at least once. This repetition is preferably performed three times. This annealing after cold plastic working is carried out only on the inner surface while cooling the outer surface, at least after the above-mentioned quenching treatment. This annealing temperature is 640
The temperature is preferably below 600°C, particularly preferably below 600°C.
The lower limit temperature is preferably 500°C. The temperature of the final annealing is preferably lower than that of the intermediate annealing, and preferably 400 to 610°C. The annealing time is preferably 1 hour or less. The annealing heating method involves placing a heating element inside the tube and cooling the outside of the tube using water, steam, gas, a salt bath, a cooling mold, or the like. It is preferable to perform annealing while creating a temperature gradient inside and outside the tube, with the inside being at a temperature higher than the recrystallization temperature of the alloy and the outside being lower than the recrystallization temperature. By annealing with such a temperature difference, the outer surface has fewer and finer precipitates than the inner surface, and the outer surface has excellent corrosion resistance against high-temperature, high-pressure water and is soft and resistant to stress corrosion cracking. Excellent quality.
Since annealing can be performed at a high temperature of 900° C. or lower, the inner surface can have a substantially perfect recrystallized structure. Furthermore, since heating of the outer surface can be prevented, the amount of precipitates can be reduced. If the annealing temperature exceeds 900°C, β phase will appear, quenching will occur during cooling, and hardening will occur, which is not preferable. 600 for the thick part from the outside to 1/3
It is preferable to keep the temperature below ℃. The cladding tube obtained by this method has a processed structure with a hardened structure on the outer surface and a substantially perfect recrystallized structure on the inner surface, and has superior nodule corrosion resistance and stress corrosion cracking resistance. have sex. FIG. 1 is a block diagram showing the locations where hardening and annealing of the present invention are performed. The quenching from the temperature range including β of the present invention is performed after hot plastic working, before annealing, and after cold plastic working in the method of 2 and 3 in the figure, in which annealing and cold plastic working are repeated. This is carried out at least once before post-annealing. In particular, it is preferable to carry out after hot plastic working and before annealing. The annealing of the present invention is a conventional method.
This is done in place of the annealing in (1), and at least 1
It can also be carried out in combination with conventional whole tube annealing. 3 in the figure is a manufacturing method that combines the conventional case where the whole tube is quenched and the annealing according to the present invention. As described above, each treatment is performed at least once, but it is preferable that quenching is performed once immediately after the final hot working, and cold working and annealing are repeated three times. The nuclear reactor fuel cladding tube of the present invention is made of a zirconium-based alloy and can also be applied to one in which a metal barrier is provided on the inner surface. Metal barriers include pure zirconium, zirconium alloys containing no tin and small amounts of iron and chromium, copper, niobium, stainless steel, nickel, and aluminum. The thickness is 5 to 15% of the thickness of the cladding tube, and it is particularly preferable to use pure zirconium. The present invention is applied to a nuclear fuel element that has a central core of a nuclear fuel material body and a cladding made of a zirconium-based alloy that holds the central core, and has a gap between the core and the cladding. FIG. 2 is a partial sectional view showing an example of a nuclear fuel element according to the present invention. The central core 26 is placed in the cladding tube 17 and includes the stud 19, the end plug 18 and the spring 2.
8. The central core may be a uranium compound, a plutonium compound, or a mixture thereof. FIG. 3 is a partial cross-sectional configuration diagram showing an example of a nuclear fuel assembly according to the present invention. Each nuclear fuel element 20 is attached to a channel 21 and inserted into a nuclear reactor. Preferably, the reactor fuel cladding tube of the present invention has an outer surface having a textured texture or a partially recrystallized texture, and an inner surface having a completely recrystallized texture. The cladding tube of the present invention is applied to light water reactors (boiling water type, pressurized water type) and heavy water reactors. [Embodiments of the invention] Example 1 The chemical components of the Zircaloy-2 ingot used were 1.43% Sn, 0.16% Fe, 0.11% Cr, and 0.06% by weight.
%Ni, balance Zr. This material is hot extruded,
A raw tube with an outer diameter of 63 mm, wall thickness of 10 mm, and length of 2500 mm was manufactured. After that, both ends of this raw tube were sealed, and induction hardening was performed while cooling the inside with circulating water.
The quenching was carried out by fixing the high frequency oscillation coil and lowering and moving the raw tube. FIG. 4 is a configuration diagram showing an example of the hardening apparatus of the present invention. Both ends of the billet or unmanufactured pipe 1 are connected to water conduit pipes 10 and 11 by flanges 7 and 8,
The inside of the raw tube 1 is constantly cooled. On the other hand, the outside of the tube is heated to reach the quenching temperature by the high frequency oscillation coil 4. By moving the upper and lower fixing plates 5 and 6 up and down, the entire length of the raw tube 1 can be hardened. Although water was used as the cooling medium in this case, a predetermined temperature gradient can also be obtained by introducing argon gas. An example of the temperature distribution during quenching temperature rise is shown in FIG.
This temperature distribution is obtained when water is used as the cooling medium. In this case, the temperature inside the tube will be 100° C. or less, but as described above, it may be heated to the upper limit temperature of the α phase region. However, if the temperature exceeds 600°C, the precipitates will become coarse and the nodule corrosion resistance will be reduced, so it is desirable to keep the temperature inside the tube below 600°C. The thermal history on the outside of the tube at this time is 960℃, 20~
It was held for 30 seconds and then cooled to below 100°C within 1 minute. The temperature inside only rose to a maximum of 100 degrees Celsius, and it only rose for a very short time. Thereafter, cold plastic working with a cross-section reduction rate of 70% was performed once at room temperature. For annealing after cold working, a high frequency oscillation coil 4 was inserted into the tube as shown in FIG. 6, and the coil was gradually moved for annealing in the longitudinal direction of the tube. At the same time, the temperature on the outside of the tube was always kept low by injecting argon gas from the cooling nozzle 14. The thermal history inside the tube at this time is approximately
The temperature was maintained at 700°C for 5 minutes, and then the temperature was cooled to below 100°C within 10 minutes. On the other hand, the temperature on the outside of the tube is approximately 500°C, which is lower than that on the inside. Note that the temperature gradient at this time is shown in FIG. Furthermore, cold plastic working was performed in the same manner as described above, and after annealing at 600°C for 2 hours, cold plastic working was performed in the same manner, and final annealing was performed at 577°C for 3 hours. During annealing, dummy tubes 12 and 12' were welded to the tube ends, and argon gas 15 was flowed inside the tube to prevent oxidation. As shown in FIG. 8, the cladding tube of the present invention had a processed structure or a partially recrystallized processed structure on the outside, and a sufficiently softened recrystallized structure on the inside. In the figure, a is the hardened part, b is the boundary between the hardened part and the annealed part, and c is the annealed part. Part b is approximately half the thickness. Thereafter, a corrosion test and an SCC test were conducted in an iodine atmosphere using these cladding tubes. The corrosion test was conducted in steam at 500°C for 24 hours, and after the test, the appearance of the test piece was observed and the thickness of the oxide film was measured. FIG. 9 shows a comparison of the corrosion resistance of the conventional pipe and the pipe of the present invention. Nodular corrosion has been observed in conventional pipes, and the oxide film thickness varies widely. In contrast, no nodular corrosion was observed in the tube of the present invention, and a uniform black oxide film was exhibited. The coating thickness of the tube of the present invention has small variations and is located at the lower limit of the variation range of conventional tubes. The conventional tube was not hardened, but was subjected to cold plastic working three times in the same manner as in the present invention, intermediate annealing at 650° C. for 2 hours, and final annealing in the same manner as in the present invention. Figure 10 shows the results of the SCC test in iodine.
The circumferential elongation after SCC crack initiation was measured under conditions of a test temperature of 350°C and an iodine concentration of about 1 mg/cm ² (relative to the inner area of the cladding tube). As is clear from the figure, the circumferential elongation of the tube of the present invention is higher than that of the conventional tube, indicating that the tube of the present invention has excellent SCC resistance. FIG. 11 is a diagram showing the relationship between annealing temperature and corrosion increase. The samples were heated at 940°C for 20 seconds, quenched by water spray, cold plastic worked at room temperature with a reduction in area of 70%, and then annealed at various temperatures for 2 hours. The chemical composition of the sample is the same as described above. Corrosion tests were conducted at 500°C for 24 hours in water vapor. It can be seen that when the annealing temperature is 600°C or higher, the corrosion weight increases. The recrystallization temperature depends on the degree of processing, but begins to occur at approximately 500°C or higher. It has been found that in order to obtain better corrosion resistance, it is preferable that the annealing temperature of the outer side in contact with high-temperature, high-pressure reactor water in the nuclear reactor does not exceed 600°C. Example 2 The material used was the same hot extruded tube as in Example 1. As shown in FIG. 12, β-quenching of the extruded tube 1 was carried out by heating the outside of the tube with a high-frequency coil 6 and bringing the inside of the tube into contact with a cooling mold 7 to radiate heat. The advantage of this cooling method is that the temperature inside the tube can be controlled by adjusting the degree of contact with the hardening material and the flow rate of water 11 that cools the mold 7. The temperature during quenching was 1000°C on the outside of the tube and 550°C on the inside of the tube. The subsequent cold plastic working and annealing were repeated three times to produce a nuclear reactor cladding tube. This example is based on the manufacturing process shown in Fig. 1 and 2, and the quenching is 1
Only once. The second and third processing and annealing are the same as the inventive production of Example 1. It was found that the nodular corrosion resistance of the cladding tube obtained by this method was superior to that of conventional cladding tubes that were not hardened. At the same time, it also showed good iodine SCC resistance. Example 3 The material used was the same hot extruded tube as in Example 1. For β quenching, integral quenching was performed. Its thermal history is
The mixture was heated and held at 1000°C for 20 seconds, and then cooled to room temperature within 1 minute. Thereafter, a first cold rolling process was performed at room temperature, followed by annealing according to the method of the present invention. FIG. 6 is a block diagram of an apparatus for annealing raw pipes according to the method of the present invention. A high-frequency oscillation coil 4 was inserted into the inside of the rolled tube 1 to heat it, and the entire tube was annealed by moving the tube in the longitudinal direction while cooling the outside with water spray from a cooling nozzle 14. The heat history at this time is 800 on the inside.
℃, 5mm, and on the outside it was below 150℃.
Thereafter, cold rolling and annealing at 600°C for 2 hours were repeated twice to obtain a cladding tube. The annealing was performed on the entire tube. The second and third processing and annealing are the same as in the inventive production of Example 1. Test pieces were cut from these cladding tubes and subjected to nodular corrosion and iodine SCC evaluation tests. Table 1 shows the test results. The comparison materials were a conventional tube that was not quenched and a tube that was entirely quenched. As is clear from the test results, the tube of the present invention has excellent nodular corrosion resistance and iodine SCC resistance.
Note that the conventional tube is manufactured using the same method as the conventional tube of Example 1 described above.

〔Effect of the invention〕

According to the present invention, a reactor fuel cladding tube having excellent nodule corrosion resistance and stress corrosion cracking resistance can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the manufacturing process of a reactor fuel cladding tube, FIG. 2 is a partial cross-sectional configuration diagram showing an example of a nuclear fuel element using the reactor fuel cladding tube of the present invention, and FIG. A partial cross-sectional configuration diagram showing an example of a reactor fuel assembly using the reactor fuel cladding tube of the invention,
Fig. 4 is a block diagram showing an example of an apparatus for carrying out the quenching method of the present invention, Fig. 5 is a diagram showing the heating temperature distribution of the tube during quenching of the present invention, and Fig. 6 is a diagram showing the annealing method of the present invention. A configuration diagram showing an example of a device for
Fig. 7 is a diagram showing the heating temperature distribution of the tube during annealing of the present invention, Fig. 8 is a micrograph showing the metal structure of the cross section of the reactor fuel cladding tube of the present invention, and Fig. 9 is a high temperature and high pressure water test. A bar graph showing the subsequent oxide film thickness, Figure 10 is a bar graph showing the elongation rate of the cladding tube,
FIG. 11 is a diagram showing the relationship between annealing temperature and corrosion increase, and FIG. 12 is a configuration diagram showing an example of an apparatus for carrying out the annealing method of the present invention. 1...Made pipe, 7...Cooling mold, 4,6...High frequency oscillation coil, 14...Cooling nozzle, 12,1
2'...Dummy, 17...Reactor fuel cladding tube,
18... End plug, 19... Include stud, 21... Channel, 22... Lifting bail, 23... Upper outlet, 20 ...
Nuclear fuel element, 24...Nuclear fuel material holding means, 25...
...Top end plate, 26...Nuclear fuel central core.

Claims (1)

[Claims] 1. After final hot plastic working of the zirconium-based alloy,
In the method for manufacturing a cladding tube, the cladding tube is subjected to a quenching treatment in which the zirconium-based alloy is heated to a β phase or α+β phase temperature range and then rapidly cooled, and then subjected to cold plastic working and annealing at least once, wherein at least the cold treatment after the quenching treatment is performed. A cladding tube for nuclear fuel, characterized in that annealing after plastic working is performed by heating an inner surface of the cladding tube to a temperature equal to or higher than the recrystallization temperature of the zirconium-based alloy while cooling the outer surface of the cladding tube. Manufacturing method. 2 After final hot plastic working of the zirconium-based alloy,
In the method for manufacturing a cladding tube, the quenching process is performed by heating and rapidly cooling the zirconium-based alloy to the β phase or α+β phase temperature range, and then cold plastic working and annealing at least once, in which the quenching process is performed on the entire cladding tube. By heating to the β phase or α+β phase temperature range and rapidly cooling, or by heating the outer surface of the cladding tube to the β phase or α+β phase temperature range and rapidly cooling while cooling the inner surface of the cladding tube. and at least the annealing after the cold plastic working after the quenching treatment is performed by heating the inner surface of the cladding tube to a temperature equal to or higher than the recrystallization temperature of the zirconium-based alloy while cooling the outer surface of the cladding tube. A method for manufacturing a cladding tube for nuclear reactor fuel, characterized by: